Because it is irresponsible to have too many kids you can't care for. It's not a reversal of the RC stance on artificial contraception. Lots of people think that the RC says NO birth control, but that is wrong; natural family planning is acceptable and encouraged. NFP is very effective as both a contraception method and a method to achieve pregnancy. It takes some education, patience, and some periods of abstinence (maybe a week or so of no sex each month), but is also very healthy, as it does not use any chemicals.